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88 Chiller System Design and Control SYS-APM001-EN
System Controls
high. The control point is selected to minimize over-pressurizing the system
and to assure adequate flow at all critical loads.
Critical valve reset (pump pressure optimization)
Often, pumps are controlled to maintain a constant-pressure differential at a
remote coil. Unless this coil serves the zone requiring the most pressure (the
critical zone), the pump provides more pressure than necessary and
consumes more power than necessary. Systems that have integrated airside
and chiller plant control systems and contain valves with direct digital
controls present an opportunity for further pump operating-cost reduction. If
a system is monitored to determine the critical valve at each point in time, the
pump’s operating pressure can be reset so that the critical zone’s control
valve is nearly wide open.
Integrated control allows a system-level controller to determine the critical
zone and reset the pump’s setpoint dynamically, therefore reducing pump
operating costs.
Air-handler controllers know the position of their individual valves, which
modulate to maintain the required water flow through the coil. The building
automation system continually monitors the valve controllers, looking for the
most open valve. The controller resets the pump setpoint so that at least one
valve, the one requiring the highest inlet pressure, is nearly wide open. The
result is that the pump generates only enough pressure to get the required
water flow through this “critical” valve, thus saving pumping energy.
Addendum ak to ASHRAE 90.1-2007 now requires pump pressure for many
chilled water systems. It will become part of 90.1-2010.
Number of chillers to operate
Some practitioners believe that operating more chillers at the same time
permits each chiller to operate more efficiently. From a system perspective,
this is rarely true. The chillers are not the sole energy consumers in the
plant—activating a chiller also activates the condenser water pump, perhaps
a chilled water pump, and tower fans. One analysis showed that if all
operating chillers are equipped with variable frequency drives (VFDs), the
only time it makes sense to run more chillers than necessary is if the
condenser water drops below 65°F. Without VFDs on the chillers, the system
never used less energy with the extra chiller operating.